This invention relates to spherical powder particles and to the process for producing the particles which involves mechanically reducing the size of a starting material followed by high temperature processing to produce fine spherical particles. More particularly the high temperature process is a plasma process.
U.S. Pat. No. 3,909,241 to Cheney et al relates to free flowing powders which are produced by feeding agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles and collecting the particles in a cooling chamber containing a protective gaseous atmosphere where the particles are solidified.
Fine spherical precious metal powders such as gold, silver, platinum, palladium, ruthenium, and osmium and their alloys are useful in applications such as electronics, electrical contacts and parts, brazing alloys, dental alloy applications such as fixed restorations, crown and bridge, amalgam alloys, and solders. Typically materials used in microcircuits have a particle size of less than about 20 micrometers as shown in U.S. Pat. No. 4,439,468.
The only commercial process for producing such metal powder particles is by gas or water atomization. Only a small percentage of the powder produced by atomization is less than about 20 micrometers to size. Therefore yields are low and powder costs are high as a result.
Therefore, a process for efficiently producing fine spherical precious metal powder particles would be an advancement in the art.
In European Patent Application WO8402864 published Aug. 2, 1984, there is disclosed a process for making ultra-fine powder by directing a stream of molten droplets at a repellent surface whereby the droplets are broken up and repelled and thereafter solidified as described therein. While there is a tendency for spherical particles to be formed after rebounding, it is stated that the molten portion may form elliptical shaped or elongated particles with rounded ends.